Rotating diffusion cell studies of microemulsion kinetics

Abstract

The rotating diffusion cell is used to study the release of H⁺ from a water-in-oil microemulsion, stabilised with Aerosol-OT (AOT), into a coexisting aqueous phase. By measuring the release of H⁺ as a function of the rotation speed, the effects of mass transport can be separated, and rate constants for the kinetics of the transfer at the interface can be determined. With increasing concentration of droplets the rate reaches a limiting value. Measurements of the drop size, using photon correlation spectroscopy, allow the geometry of and the number of protons carried by each droplet to be found. The kinetic results are explained by a model in which the droplets first adsorb on the interface according to a Langmuir isotherm, and then in the rate-limiting step merge with the aqueous phase. A simple theoretical model is presented which compares this heterogeneous process with the equivalent homogeneous reaction, in which H⁺ is transferred between two droplets in the same phase. Again the process is separated into an association step, followed by a rate-limiting fusion of the droplets. In either case the association pre-equilibrium is found to be larger than the statistical value, and the rate-limiting fusion has a first-order rate constant in the range 3 × 10² to 10³ s^–1.